The present invention relates generally to glass sheets and more specifically to sintered glass sheets such as silica glass sheets made using a glass soot deposition and sintering process.
Glass sheet materials can be formed using a variety of different methods. In a float glass process, for example, a sheet of solid glass is made by floating molten glass on a bed of molten metal. This process can be used to form glass sheets having uniform thickness and very flat surfaces. However, float glass processes necessarily involve direct contact between the glass melt and the molten metal, which can lead to undesired contamination at the interface and less than pristine surface quality. In order to produce high quality float glass sheets with pristine surface properties on both major surfaces, float glass is typically subjected to surface polishing steps, which add additional expense. Moreover, it is believed that the float process has not been used to make ultra-thin, rollable glass sheets.
An additional method for forming glass sheet materials is the fusion draw process. In this process, molten glass is fed into a trough called an “isopipe,” which is overfilled until the molten glass flows evenly over both sides. The molten glass then rejoins, or fuses, at the bottom of the trough where it is drawn to form a continuous sheet of flat glass. Because both major surfaces of the glass sheet do not directly contact any support material during the forming process, high surface quality in both major surfaces can be achieved.
Due to the dynamic nature of the fusion draw process, the number of glass compositions suitable for fusion draw processing is limited to those that possess the requisite properties in the molten phase (e.g., liquidus viscosity, strain point, etc.). Further, although relatively thin glass sheets can be made via fusion draw, the process cannot be used to form ultra-thin, rollable high-silica glass sheets. Finally, the apparatus used in the fusion draw process can be expensive.
In addition to their limitations with respect to ultra-thin glass sheet materials, both float and fusion draw processes are largely impractical sheet-forming methods for high-silica glass sheets due to the high softening point (˜1600° C.) of silica. Rather, silica glass substrates are typically produced by cutting, grinding and polishing silica ingots produced in batch flame-hydrolysis furnaces. Such a batch approach is extremely expensive and wasteful. Indeed, the requisite slicing and polishing that would be needed to produce uniform, thin, flexible silica glass sheets via flame-hydrolysis render the process prohibitively expensive. Using known methods, Applicants believe that it is not currently feasible to form and polish both sides of a high-silica glass sheet having a thickness of less than 150 microns.
In view of the foregoing, economical, uniform, ultra-thin, flexible, rollable high-silica glass sheets having a high surface quality are highly desirable. The high-silica glass sheets can comprise one or more layers, components, or phases. Such glass sheets can be used, for example, as photo mask substrates, LCD image mask substrates, and the like.
A high-silica glass sheet has two major opposing surfaces having an average thickness of 150 microns or less and an average surface roughness over at least one of the two major surfaces of 1 nm or less. In an embodiment, an average surface roughness over both of the two major surfaces is 1 nm or less. Example high-silica glass sheets measure at least 2.5×2.5 cm2. For example, a width of the glass sheet can range from about 2.5 cm to 2 m and a length of the glass sheet can range from about 2.5 cm to 10 m or more. Indeed, the length of the glass sheet is limited in principle only by the deposition time, and can extend beyond 10 m to 10 km or more. The glass sheet is formed using a roll-to-roll glass soot deposition and sintering process. In a further embodiment, the glass sheet comprises a plurality of pseudo-visible striations. The striations are caused by local thickness variations and are visible only when a major surface of the glass sheet is viewed at an angle sufficiently removed from normal incidence (e.g., greater than 5 degrees).
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.